A handwritten note stating why you think your work would be a good fit doesn't hurt either. There, all wrapped up like a cozy little illustration package.
Print the addresses right from my printer onto the manila envelope, and added a little sketch to grab the viewers attention (hopefully). I used my 2013 copy of the "Children's Writer's & Illustrator's Market" to find which publishers I want to submit to. I only use the book to find publishing companies looking for illustrators. Always go to a publishers website to get the submission information. If possible, I will also check the art director's name on Twitter or Linkedin to see if they still work for that publisher.
I have these four little characters incorporated into my promotional stuff. So for each sketch on the front of the envelope, the corresponding sticker on the back to seal the contents safe inside. And done! Easy, relatively affordable, and personal.